This invention relates to a method for forming an angled plastic article of varying density and, more particularly, to a method for molding and bending a plastic article to form a smooth dense outer shell and a rare interior.
Injection molding machines generally include a two-section mold unit wherein one of the mold sections is stationary and includes an end gate opening for the injection of mold material into a mold cavity formed by the two mold sections. The other mold section is generally movable between an open position away from the stationary mold section and a closed position wherein the two mold sections are in sealed contact engagement to form the mold cavity. Once the mold cavity has been formed, a screw or similar injection device is used to inject a plastic material into the mold cavity where the material is cured under pressure. Injection molding provides an efficient means for producing plastic articles both quickly and economically.
It is often desired to create a plastic article of a decreased density which reduces the amount of material required to make the part and which significantly decreases the weight of the part. One way of creating a plastic part of reduced density is through the use of a blowing agent. Blowing agents are well-known in the art with most blowing agents being heat-activated. When mixed with a plastic material under controlled conditions these blowing agents produce bubbles. If a blowing agent is evenly distributed throughout the plastic material the bubbles are generally trapped evenly throughout the plastic article as the article cures. These trapped bubbles form a cellular structure within the finished article.
While it is often desirable to have a cellular structure within a plastic article, this cellular structure is typically undesirable on the outer shell of a finished plastic article. Bubbles forming near the outer shell of a curing plastic article often result in unaesthetic holes appearing on the surface of the article. Such surface bubbles may also compromise the integrity of the surface by creating sink holes or weak points where the article is unable to sustain pressure.
As described in co-pending U.S. patent application Ser. No. 08/082,266, now U.S. Pat. No. 5,437,823 it is desirable to mold a plastic part with a uniform cellular structure on the interior, which allows the part to be molded lighter and with less material, while at the same time having a denser outer skin, which increases the aesthetic and structural aspects of the finished article. To mold a plastic article without surface defects, the co-pending application describes a process whereby a two-section mold unit is filled with a plastic injection material having a blowing agent incorporated therein. As the injection material contacts the cool sides of the mold unit, the blowing agent is deactivated and the material near the sides of the mold cavity hardens into a smooth exterior. The warm interior of the material, however, has a cellular structure caused by the heat activation of the blowing agent within the injection mixture.
Because an article produced by the above process produces a lightweight article with a smooth aesthetic exterior appearance, it would be desirable to produce large containers and other objects which could benefit from the weight decrease of the interior cellular structure. Given that this process requires that the mold units move in relationship to one another during the molding process, the process is very well suited for producing flat sheets of material. While it is possible to produce more complex shapes with the above process, the mold units for producing such articles would necessarily be larger and more complicated owing to the requirements of moving the mold sections in relationship to one another during the molding process.
Another difficulty associated with molding a plastic article with a rare cellular interior and a smooth plastic exterior is the difficulty in maintaining an exterior skin of a constant thickness around comers in the mold cavity. Typically, in a molding process, the increased cooling effect of the mold cavity at places where two sides of the mold cavity meet leads to a thicker skin being formed at that point. This added thickness not only increases the amount of material which must be added to the mold cavity, thereby increasing costs, but the increased thickness also adds additional weight to the finished plastic article. Additionally, because the necessity of moving the mold sections in relationship to one another, and due to the limitations of mold cavity shape in plastic injection processes, it has not been possible to mold certain shapes with a cellular interior and smooth exterior. It would therefore be desirable to mold a flat sheet which could then be transformed after molding into a more complex shape.
The difficulties encountered in the prior art discussed hereinabove are substantially eliminated by the present invention.